Crosshair-
You're reading way too much into it. Open bolt firearms are not necessarily NFA items. The Spitfire carbine was DECLARED a machinegun in 1968 as Mapsjapan's link shows. That is the same info in the 1968 pamphlet. They were not declared a machinegun simply because they were open bolt firearms.
They had a design (flaw) that allowed them to be fired automatically.